The present invention relates to a pneumatic tire, particularly to a low aspect radial tire for passenger vehicles, capable of providing higher cornering power so as to improve steering stability, while enhancing dry and wet grip performances and reduction of pass-by noise.
Field of the Invention
Recently, as automobiles become more quiet, tire noise now contributes at a higher ratio to the total noise level of the automobile. Thus, a reduction in tire noise is demanded. Such noise reduction is specifically desired in a range of around 1 kHz, which forms the peak frequency of tire noise, and sounds due to columnar resonance generated by the circumferential grooves is one of the main sound sources in such a high frequency range.
On the other hand, in order to maintain wet grip performance, the tread of a tire is generally provided with a plurality of circumferential grooves continuously extending in the circumferential direction of the tire.
In such a tire, when it is in contact with the ground, a kind of air column is formed by the road surface and the circumferential groove. A sound of specific wavelength, which is double the wave length of the air column is caused by the airflow within the column during running.
Such a phenomenon is referred to as a columnar resonance, and provides the main source of noise at 800 to 1.2 kHz. The wavelength of the columnar resonance sound is approximately constant, giving a constant frequency regardless of the tire's speed, and thus increases the sound inside and outside of the automobile. Incidentally, since this noise of around 1 khz is a sound easily heard by the human ear, the increase of noise with this frequency greatly influences tire noise performance.
Although reduction of the number or volume of the circumferential grooves is known to prevent columnar, such reductions lead to a lower wet grip performance.
On the other hand, although wet grip performance can be increased by increasing the number or volume of circumferential grooves, a simple increase causes a reduction of the dry grip performance, because the ground-contact area is reduced. Also, this causes a reduction in steering stability as the rigidity of the tread pattern is reduced, in addition to an increase in tire noise.
To achieve such contradictory performances, it has been attempted to satisfy both wet grip performance and steering stability by broadening the groove width of the circumferential groove, and arranging the number, such as two or four, about the tire equator.
In a tire of such a constitution, however, when the load acting on the tire shifts in the tire axial direction at the time of cornering, since the groove volume is large, the ground contact pressure increases abnormally at the load shifting side, and the steering stability in cornering, or the so-called turning steering stability becomes inferior.